As a recording medium for recording information such as documents, sounds or images, a recording medium employing e.g. a magnetic material, a magneto-optical material, an organic dye material or a phase change material composed of an inorganic material, has been known.
In such a recording medium, a physical parameter of a recording layer such as refractive index, electric resistance, shape, volume or density, is changeable by applying an external factor to the recording layer by such a method as heating the recording layer by light irradiation or electric current through the recording layer or application of magnetic field to the recording layer. Further, in such a recording medium, usually, the difference of such a physical parameter between before and after the application of external factor to the recording layer is used for recording or reproducing an information.
As an example of such a recording medium, an optical recording medium recordable by irradiation of laser beam is mentioned. Such an optical recording medium is further roughly categorized into a write once type optical recording medium to which one time of recording is possible but rewriting is not possible, and a rewritable optical recording medium to which repeated recording is possible. Among these optical recording media, in recent years, a write once type optical recording medium is attentioned for such reasons that it is suitable for recording of e.g. a public document whose interpolation is not allowed, it is suitable for high speed recording, and it can reduce production cost.
For a recording layer of such a write once type optical recording medium, various types of constructions are proposed, which include one employing an organic compound, a phase change type employing an inorganic material, an alloy type or a perforation type. Among these, a construction disclosed in Patent Document 1 which has a thin recording layer containing an inorganic material discharging a gas so that the gas is discharged by heat of laser irradiation and deformation caused by the gas discharge is used for recording, is prospective for the reasons that it produces large signal amplitude and it has high irreversibility of preventing repeated recording.
According to Patent Document 1, by constructing a recording layer containing a material that is decomposed at an arrival temperature of the recording layer by heat at a time of recording, and a material undergoing no chemical reaction or phase change at the temperature, a write once type optical recording medium is obtained, which has a wide recording power range in which good recording signal characteristics is obtained.    Patent Document 1: JP-A-2006-18981